Siloxane polymers or copolymers having a high refractive index have been increasingly used for a variety of optical applications including, for example, in contact lenses, intraocular lenses, etc. Such polymers are also finding their way into other optical applications requiring high transmission and high refractive index including but not limited to, solid state lighting (light emitting diodes, organic light emitting diodes, laser diodes), waveguides (both planar and “fiber” geometries), optical computing, optical storage media, antireflection coatings, conformal coatings, optical lenses, micro lenses, automobile topcoats, paint formulations, hair care products, gradient refractive index optical components, dynamic gradient refractive index components, etc.
Depending on the application, the polymers and products formed from such polymers may need to exhibit a wide range of properties including sufficient structural integrity, strength, elasticity and elongation, thermal stability and index of refraction. In some applications, the polymers must exhibit these properties when formed into a thin film. For example, in intraocular lenses, the lens must be thin and pliable for insertion through a small incision in intraocular lens applications, be able to regain its original shape after incision, and have sufficient structural integrity and strength to retain such shape under normal use conditions.
Typical optical grade methyl siloxanes have excellent optical transmission, but intrinsically low refractive index (1.41) and poor barrier properties against moisture and gas. The development of higher refractive index siloxanes with improved barrier properties has revolved around the use of cyclohexyl, cyclopentyl, and phenyl groups to increase the refractive index beyond 1.40. Examples of typical optical grade siloxanes include a copolymer of cyclohexylmethyl-dimethyl siloxanes, cyclopentylmethyl-dimethyl siloxanes, diphenyl-dimethyl siloxanes, or methylphenyl-dimethyl siloxanes.
Aromatic groups are traditionally introduced into the siloxane polymers and conventional co-polymers to increase the refractive index of the materials. Most often these groups consist of dimethylsiloxane-phenylmethylsiloxane co-polymers or dimethylsiloxane-diphenylsiloxane co-polymers. At a phenyl content of approximately 15 mole %, a polydimethyl siloxane/methylphenyl siloxane co-polymer has a refractive index of 1.462.
The introduction of refractive index modifying groups, such as phenyl-groups, in polysiloxanes is known to result in several disadvantages. Materials formed from siloxanes containing phenyl groups can have reduced flexibility, poor mechanical strength and elasticity, and they may be hard and brittle. Further, materials with phenyl content greater than 40 wt % are not easily processed and tend to exhibit poor mechanical strength. This limits the refractive index that can be achieved to about 1.54. In addition, phenyl group containing polymers are known to be unstable in ultraviolet light.
The incorporation of phenyl into the silicones makes the resulting polymer more vulnerable under thermal and UV exposure conditions. This result in yellowing of the optical material and transmission losses such that the transmission level is below a tolerable level and can lead to mechanical failure of a device in the optical components. There is a need for high refractive index siloxanes that have good physical properties and are thermally stable.